The use of aqueous dispersions of photographic couplers and other hydrophobic photographically useful compounds is known in the art. Generally, dispersions of hydrophobic photographically useful materials (PUMs) in aqueous solutions are prepared by one of the following ways: milling of solid particles using the well known methods of comminution; precipitation of photographically useful materials from solution; and homogenization of a liquid organic phase containing a photographically useful material into an aqueous solution containing a hydrophilic colloid such as gelatin and, optionally, a surface active material.
Processes for homogenization of liquid organic phases frequently include the use of low boiling or at least partially water miscible auxiliary solvents, which auxiliary solvent is subsequently removed after homogenization by evaporating volatile solvent or washing water miscible solvents. Such auxiliary solvents facilitate combining couplers and/or any other hydrophobic dispersion components in a mixed solution, so that a dispersion with an oil phase of uniform composition is obtained. The solvent also lowers the viscosity of the oil solution, which allows the preparation of small-particle emulsified dispersions. The use of auxiliary solvent may also be used to form a liquid organic solution of a PUM for forming a dispersion where no permanent solvent is desired in the final dispersion. However, the use of auxiliary solvent also presents several difficulties in the preparation of photographic dispersions and elements. Auxiliary solvents can cause severe coating defects if not removed before the coating operation. Also, it is not possible, due to thermodynamic considerations, to remove 100% of the auxiliary solvent from the dispersion. This may cause other deleterious effects such as enhancing the solubility and movement of the PUM, or aid in crystallization. Further, the steps of evaporating volatile solvent from an evaporated dispersion and washing a chill-set, washed dispersion often leads to final photographic dispersions with variable concentration, so that careful analysis is necessary to determine the actual concentration of the photographically useful compound in the dispersion. Volatile or water-soluble auxiliary solvents present health, safety, and environmental hazards, with risks of exposure, fire, and contamination of air and water. The cost can be significant for the solvent itself, as can be the costs of environmental and safety controls, solvent recovery, and solvent disposal.
Alternatively, PUMs may be "directly" homogenized or dispersed into an aqueous solution in the substantial absence of any auxiliary solvent (i.e., absence of such solvents beyond trace or impurity levels). In one such direct dispersion process, the hydrophobic components desired in the dispersion, e.g., coupler and permanent coupler solvent, are simply melted at a temperature sufficient to obtain a homogeneous oil solution. This is then emulsified or dispersed in an aqueous phase, typically containing gelatin and surfactant. The direct process also yields a dispersion with a known concentration of the photographically useful compound, based on the components added, with no variability due to evaporation or washing steps. No volatile or water-soluble organic solvents are needed, eliminating the hazards and costs associated with their use. Additionally, the absence of auxiliary solvents in the dispersion forming step generally allows for higher concentrations of permanent organic phase (comprising the photographically useful materials and any high boiling permanent organic solvent) in the resulting dispersion.
While the direct dispersion process may be preferred for the above reasons, direct dispersion formulations may result in dispersion viscosities which may be higher than desired, especially where relatively high permanent organic phase concentrations are employed (e.g., above 16 weight percent). Such high viscosities may cause difficulty in pumping and filtering, e.g., which may lead to higher waste. Where the dispersion viscosity limits the viscosity of a photographic layer coating composition, subsequent operations for coating the dispersion, deaeration and the coating process itself may be adversely affected. It is typical to manage this viscosity by diluting the dispersion. Increased amounts of water in a coating are undesirable, however, because drying the coating may be more difficult. While lower dispersion viscosities are desirable for ease in pumping, filtration, dearation, coating and drying, they are generally difficult to achieve at high organic phase concentrations.
As indicated above, the aqueous phase of direct dispersions typically contain gelatin. Gelatin used in forming photographic dispersions and elements is available in various forms such as alkali-treated gelatin, acid-treated gelatin, and gelatin derivatives manufactured by treating or reforming gelatin in various ways. Such gelatins generally have an average molecular weight of from tens of thousands to several hundred thousands, as determined by terminal group analysis, amino acid composition analysis, light scattering, gel permeation, ultracentrifuging, or surface pressure measurement. Dispersion viscosity is a function of the size and concentration of the gelatin molecules employed in the dispersion. The viscosity of gelatin solutions is particularly sensitive to the fraction of the gelatin molecules which have a molecular weight greater than 150,000, which typically constitute the .gamma. fragment and microgel fragment of hydrolyzed collagen (from which gelatin is obtained). Cattle (cow) bones are the principal starting material for gelatin typically used in photographic elements. While cattle and pig skins may also be also be used, skin gelatins usually contain photographically active components, and their uses in photographic systems are therefore limited.
The manufacture of gelatin involves several stages. The first step is the deashing process to reduce the calcium (mainly calcium triphosphate or calcium apatite and calcium carbonate) content of the bones through a soak for about a week in a mineral acid bath. This decalcified material is referred to as collagen or "ossein." Collagen or the ossein is a crosslinked and structured polypeptide which is further treated either by lime or by a mineral acid to hydrolyze and denature the tertiary, secondary and partly the primary structures to produce water-soluble gelatin. During the formation of gelatin collagen, which is composed of crosslinked triple helices of .alpha.1 and .alpha.2 chains (MW=285,000), is first denatured to the randomly coiled .lambda. form, then to a mixture of the .gamma.11 (composed of two .alpha.1 chains MW=190,000), .gamma.12 (composed of one .alpha.1 and one .alpha.2 chain, MW=190,000), and to single .alpha.1 and .alpha.2 stands (MW=95,000) and sub-alpha fragments (MW&lt;95,000). The solubilized gelatin fractions are leached and, for many applications, deionized by passage through ion exchange beds, chilled, noodled, and then dried for storage. Lime processing to produce gelatin requires between 2 to 3 months of treatment, whereas acid treatment usually requires only several days. Consequently acid processing is typically less expensive than lime processing and thus economically attractive. As acid hydrolysis occurs more rapidly, however, it is less controllable, and typically leads to gelatins that usually have much lower average molecular weights than those derived from lime treatment. Conventional lime processed ossein gelatin typically used in photographic materials usually contains greater than 40% by weight of molecules having a molecular weight greater than 150,000 daltons, as measured by size exclusion chromatography. While relatively lower molecular weight lime processed gelatins are also available (e.g., gelatins having less than 40% by weight of molecules having a molecular weight of greater than 150,000), such gelatins have found limited use in photographic elements due to their resulting physical properties. Acid processed ossein and pigskin gelatins typically contain less than 35% by weight of such high molecular weight molecules.
The physical properties of gelatin, such as the isoelectric point (designated pI, which is the pH at which the gelatin exhibits a neutral charge), molecular weight and molecular weight distribution depend upon the nature of the processing, such as lime or acid, as discussed above. It has generally been noted that the pI of lime processed ossein (LPO) gelatin is typically in the range of pH 4.8-5.1, while that of acid processed pigskin (APP) gelatin is typically much higher at around pH 9. Acid processed ossein (APO) gelatins typically exhibit higher pI values than lime processed gelatins, but typically not as high as acid processed pigskin gelatin, e.g., in the range of 6.0-8.0. While the use of acid processed gelatins in photographic dispersions and elements would offer a cost advantage, they generally exhibit lower than desired molecular weights for photographic use, as reducing the MW of gelatin can lead to undesirable photographic element layer coating properties. Also, it has been found that pure APO gelatin in concentrated dispersions leads to viscosity increases with time at standard operating temperatures, which situation can complicate manufacturing conditions, such as requiring dilution of the dispersion or increased operating temperatures. Such practices may result in undesired increased wet load, lower throughput, coating nonuniformity and chemical instability.